A light across the ship (5)
I believe the answer is:
abeam
'across the ship' is the definition.
(nautical term)
'a light' is the wordplay.
'light' becomes 'beam' (beam is a kind of light).
'a'+'beam'='ABEAM'
(Other definitions for abeam that I've seen before include "neither fore nor aft?" , "At right angles to the side of a ship" , "To the right or left of the centre of the ship" , "from side to side on voyage?" , "at right angles to vessel" .)